(1) Field of the Invention
This invention relates to a process for the production of urethane-modified polyisocyanurate foams excellent in flame retardance and adhesion property.
(2) Description of the Related Art
It is known to produce a polyisocyanurate foam by reacting a polyisocyanate in the presence of trimer catalysts. However, the polyisocyanurate foam has extremely high brittleness, so it is generally converted into a urethane-modified polyisocyanurate foam, in which urethane linkages have been introduced by making use of a polyol.
Upon the production of a urethane-modified polyisocyanurate foam, a polyisocyanate component and a polyol component are generally formulated in such a proportion that an NCO/OH equivalent ratio is at least about 2.0. Effects improving the brittleness of the polyisocyanate foam became greater as the amount of the polyol to be used increases. The increase of its amount is accompanied by a problem of lowered flame retardance on the contrary.
On the other hand, it is known to permit the improvement of the brittleness without lowering the flame retardance when an aromatic polyester polyol is employed as a polyol component because there is little potential problem that the flame retardance is degraded even when a relatively large amount of the polyol is used. For this reason, the urethane-modified polyisocyanurate foam making use of the aromatic polyester polyol as the polyol component has been used widely in thermal insulating materials such as so-called continuous laminate boards obtained by expanding it to unite with a surface material such as a plasterboard. When the urethane-modified polyisocyanurate foam is expanded on a surface material so as to unite them like the continuous laminate board or the like, the surface material is generally heated to permit good adhesion of the foam to the surface material and to complete the trimerization of the polyisocyanate component. It is however difficult to control the surface temperature of a substrate (a material intended to be subjected to spray foaming) in the spray application in which a polyisocyanate component is mixed with a polyol component containing a trimer catalyst and other additives, etc. in situ in the construction field of a building, or the like and the resulting mixture is sprayed on a surface material such as a wall material or a ceiling material to expand the foaming ingredient therein, thereby uniting them. In addition, when spray foaming is conducted so as to give a foam having a relatively thin thickness, the heat of reaction in a foamed layer is absorbed in the substrate and the temperature of the foamed layer hence tends to be lowered, whereby the degree of completion of the trimerization becomes lower. It is therefore difficult to sufficiently exhibit the flame retardance characteristic of the urethane-modified polyisocyanurate foam. Furthermore, the trimer reaction is later than the urethane reaction when conducting the expansion at a low temperature, thereby undergoing two-stage expansion. Therefore, creep occurs right before the curing of the foam and at the same time its curing becomes slow. The adhesion property of the foam to the substrate is hence degraded to a great extent. For this reason, there has also been a problem that impossibility is encountered on the spray foaming of a urethane-modified polyisocyanurate foam on the surface of a substrate as the temperature of the substrate is lowered to 10.degree. C. or lower in a construction field or the like in winter.